The present invention relates to a control apparatus for controlling a stepless transmission for vehicles, and more particularly, to improvement of acceleration operation and deceleration operation of the transmission in a transitional period.
Conventionally, a control apparatus which steplessly changes engine output transmitted via a hydraulic coupling and transmits the engine output to wheels, as disclosed in Japanese Patent Application Laid-Open (Kokai) No. 2-3751, has been known as "a V-shaped belt type control apparatus for stepless transmission for vehicles".
In this type of stepless transmission control apparatus, a target value of a control variable such as revolutional frequency of a primary pulley (a pulley closer to an engine side) or change speed ratio is set, and the control variable is controlled toward the target value.
Japanese Patent Publication (Kokoku) No. 3-27791 shows a case where a driver accelerates vehicle speed with this type of stepless transmission. The target value of a predetermined control variable (e.g., for the revolutional frequency of a primary pulley) is set to be large, and the revolutional frequency of an engine or the reduction ratio of pulley revolution is increased following the target value. As a result, the revolution ratio of the stepless transmission varies toward a reduced side. The problem is that the engine having large inertia moment wastes a large part of the increase of output torque within a short period after the acceleration operation, without using much of the output for the driving force of the vehicle. The driver cannot attain enough of the acceleration operation nor feels the speed has been fully increased.
For the above reason, Japanese Patent Publication 3-27791 sets a target value of transitional-period control variable which increases in accordance with at least one of the increase rate of an acceleration operation amount (e.g., an amount indicative of stepping on the accelerator) and that of the vehicle velocity.
In Japanese Patent Publication 3-27791, when a predetermined delay time has passed after the acceleration operation is initiated, a target value is increased in a stepwise manner by a step change rate based on a map and the like. Thereafter, the target value is linearly increased in an increase rate of inclination obtained by a predetermined calculation. Under this control, the target values of the transitional period can be made lower than those of stable operation time, preventing excessive increase of engine revolutional frequency, and improving the acceleration operation.
However, there are limits on precise setting of the target value in Japanese Patent Publication 3-27791, since even though it also uses various detection signals for the transmission or the status of the engine, the apparatus basically sets a target value based upon the map or the calculation. More specifically, hydraulic pressure supplied to the input-side pulley and the output-side pulley, oil temperature, variations in hydraulic pressure controlling valves as products and the like influence a reduction ratio of speed of the stepless transmission, causing difficulty in precise setting of a target value. Further, the apparatus determines an increase rate of a target value of transitional period control variable in accordance with a target value of stable period control variable just before the velocity change operation is initiated. Namely, the apparatus determines the target value without consideration to a target value of the stable period control variable during a transitional period. Accordingly, the target value of the transitional period control variable determined based on the above increase rate differs from a target value of stable operation control variable. Also, the target value of transitional period control variable may be inappropriate.